This invention relates to a piston apparatus, such as an engine, having opposing pistons rigidly connected together.
Apparatuses employing reciprocating pistons within cylinders connected to rotating shafts by cranks are typically employed as parts of internal combustion engines, external combustion engines and pumps. The commonest arrangement employs one or more pistons each slidably received within a cylinder. These apparatuses have rotating shafts provided with cranks. A piston rod is rotatably connected to the crank at one end and is hingedly connected to a piston at the other end.
Such apparatuses, while very common, have the disadvantage that the force transmitted between the piston and the crank along the piston rod is not along the longitudinal axis of the cylinder, but rather is applied at an angle according to the particular angular position of the piston rod. The force may thus be resolved into a component applied parallel to the direction of travel of the piston and a component perpendicular to the direction of travel. The perpendicular component tends to push the piston against the wall of the cylinder, increasing friction and wear of the internal cylinder surface. This results in the well-known fact that such cylinders tend to become oval-shaped over a period of time and eventually have to be rebored to restore the circular cross-section.
Attempts have been made to provide engines in which the force exerted by each piston rod is essentially along the longitudinal axis of the cylinder. These attempts have resulted in variations of a type of engine which may be referred to as the "Scotch yoke engine". By way of example only, such engines are disclosed in U.S. Pat. No. 4,013,048 to Ritz and U.S. Pat. No. 3,517,652 to Albertson. In these engines the cylinders are arranged in opposite pairs. The piston rods are supported by bearings which permit only movement along the longitudinal axis of the cylinders. The piston rods are both connected to a yoke housing which has a slot extending perpendicular to the cylinder axis. The crank is connected to a slider received within the slot. Consequently, there is no sideways force on the cylinders, this being rather absorbed by the bearings supporting the piston rods.
Scotch yoke type engines however offer disadvantages which have so far prevented widespread adoption to replace conventional piston engines. The yoke housing is supported by the piston rods and reciprocates therewith along the cylinder axis. In order to withstand the forces involved, this yoke housing must be relatively heavy and considerably increases the weight carried by the pistons and consequently the inertial forces upon the pistons. In practise it has not been easy to design a yoke which is sufficiently light and yet strong enough and compact enough for the purpose intended. It should also be noted that the type of engine still provides a sideways force which is transferred to the bearing supporting the piston rods. Thus wear of these bearings becomes a problem.